The present invention relates to medical electrodes which are applied to a patient""s skin for monitoring biopotentials and, in particular, to electrodes that penetrate the patient""s skin to make the skin more electrically permeable.
Diagnostic tests, treatments and the presence of illness require obtaining and monitoring electrical signals generated by the physiological functioning of a patient. Typical electrical signals or biopotentials that are commonly monitored are those producing electro-cardiograms (EKG) from the heart, electroencephalograms (EEG) from the brain and electromylograms (EMG) from muscles. Such signals are of relatively low level and may be very weak, such as the 100 microvolt or less signals present in an electroencephalogram (EEG). The frequency range of the signals extends from 0.05 for electro-cardiograms to 3000 Hz for brain stem evoked potentials.
Skin mounted monitoring electrodes are typically used to obtain the foregoing biopotentials. The human skin is made of three distinct layers; stratum corneum, viable epidermis, and dermis. The outer 10-15 micrometers of skin, called the stratum corneum, is dead tissue that forms the primary barrier for the body. The stratum corneum is the major contributor to skin impedance and to a reduction in biopotential signal magnitudes as well as a major factor in the signal to noise ratio characteristics of skin-mounted electrodes. Below the stratum corneum lies the viable epidermis (50-100 micrometers). The viable epidermis is comprised of living cells, but contains few nerves and is devoid of blood vessels. Penetration of the skin to the viable epidermis is painless since the nerves are found in deeper tissues. Below the viable epidermis is the dermis. The dermis forms the bulk of skin volume and contains living cells, nerves and blood vessels.
Difficulties often arise when measuring weak biopotentials with skin mounted electrodes. One problem is that the outermost layer of skin has a high electrical impedance. High electrical impedance reduces signal magnitude so that a data signal may be difficult to obtain when electrical noise is present.
Noise may also be injected in the biopotential signal due to movement of the skin. This results in a variation in the skin impedance. This variation in skin characteristics causes electrical noise Which is not easily separated from the biopotential data signal of interest. If the signal to noise ratio is sufficiently low, due to skin impedance or movement artifacts, it can mask or hinder correct analyses of a patient""s condition.
A common practice is to abrade the stratum corneum prior to applying the biopotential electrode so as to lessen the skin impedance. The tissue so removed and skin oils are rinsed away, as with alcohol. The abraded skin is covered with an electrolytic paste and the electrode applied to the patient.
However, this procedure is time consuming, particularly when several electrodes are to be applied and is inconvenient in many clinical situations such as preparing the patient for surgery.
Piercing the skin to reduce its impedance has been suggested. A device for doing this is disclosed by Lewes et al U.S. Pat. No. 3,505,993. The Lewes et al electrode is a rigid metal sheet having an arrangement of projections or spikes. The spikes are approximately 1.5 millimeters in length, and have a base diameter of approximately 1 millimeter. The spikes press into the skin when applied, and the electrode is held in place by a rubber band or other similar means. A disadvantage of the electrode disclosed in Lewes et al is that the spikes are rather large and could be uncomfortable for the patient, as well as cause undue laceration to the skin should the electrode move with respect to the skin.
Gadsby et al U.S. Pat. No. 5,309,909 discloses a combined skin preparation and monitoring electrode wherein a plurality of tines are located on a resilient dome. The dome is held in place with an adhesive. When the dome is depressed toward the patient""s skin, the tines penetrate the skin, and when pressure on the dome is released, the tines retract from the skin. Gadsby et al is disadvantageous in that one must remember to depress the resilient dome or impaired readings will be obtained. Further, this electrode requires the use of an ionic gel, and cannot be used dry.
Fendrock U.S. Pat. No. 5,305,746 discloses a disposable self-prepping electrode which utilizes an array of flexible, plastic tines which serves to part high impedance outer layers of skin to expose the low impedance, blood enriched layers without scratching or abrading. The tines are imbedded in a conductive gel layer. A center electrode stud contacts the gel and makes an electrical connection to the monitoring apparatus. The tines are between approximately 0.64 to 2.8 millimeters in length, and readily flex upon application to the skin. The Fendrock electrode is disadvantageous in that it requires use of a gel layer, and the tines may not adequately penetrate the skin due to their flexibility.
Rau U.S. Pat. No. 4,685,466 discloses a measuring sensor for biomedical signals wherein one or more short needle points penetrate the stratum corneum into approximately 10-15 cell layers from the skin surface. The Rau device does not use electrode paste or jelly. Weak signals may be improved by attaching a micro-miniaturized semiconductor arrangement directly to the electrode for active impedance matching and/or amplification. Rau is disadvantageous in that it requires a separate electrical connection for each needle. Thus, only 1-5 needles are used and the biopotential signal is extremely localized. Further, the amplification means is cumbersome because it is not part of the medical electrode itself.
It is the object of the present invention to provide an electrode suitable for mounting on the skin of a patient and which provides an improved signal to noise ratio to the bioelectrical signal obtained by the electrode.
Another object of the present invention is to provide such an electrode which may be used without preparing the skin prior to application, and which does not require paste or gels.
A still further object of the present invention is to provide an electrode which can amplify weak bioelectrical signals.
The bioelectrical signal electrode of the present invention reduces skin impedance and movement artifacts and thus increases signal quality. The reduction/increase is obtained by the electrode through use of a spike means to penetrate the skin into the viable epidermis. The carrier for the spikes is sufficiently large that the electrode can be stably applied to the patient""s skin.
Thus, and in accordance with one aspect of the invention, a bioelectric signal electrode is provided with a plurality of electrically conductive spikes that are capable of penetrating the skin to the viable epidermis layer. For this purpose, the spikes may have the necessary degree of sharpness. These spikes are attached to a carrier that allows the electrode to be applied to the surface of the human body. The bioelectrode may be kept in position using an adhesive tape or an adhesive on the periphery of the carrier. Or an electrolytic adhesive may be disposed on the underside of the carrier.
The electrode spikes range from 50-250 micrometers in length. The spikes are used in arrays of 100-10,000 spikes spaced 50-250 micrometers apart. It may be desirable to vary the spike length so that various depths of the viable epidermis layer are contacted. The base of the spikes are up to 35 micrometers in width. Sharpness in the spikes facilitates skin penetration. The size of the carrier for the spikes is typically about 25 square millimeters.
If desired, the electrode can be used with a conductive gel located between the skin and the carrier. However, a conductive gel is not always necessary, and the bioelectrode may be used dry. If conductive gel is used, it may contain antibacterial agents to prevent irritation or infection due to spike penetration of the skin.
In an alternative embodiment of the invention, the spikes are electrically nonconductive, and extend through a conductive element on the lower side of the electrode. The non-insulated spikes make the skin more permeable. An electrolytic gel or paste is used in conjunction with an electrode so constructed to make movement artifact less prominent and improve the signal to noise ratio.
When used to monitor weak biopotential signals, the bioelectric signal electrode having the aforesaid array of spikes is applied to a patient""s skin so that the spikes penetrate the skin to the viable epidermis layer. Electrical leads are attached between the electrode and a monitor so that the biopotential obtained from the electrode may be amplified for display or recording.
In yet another embodiment of the present invention, an amplifier and battery are attached to the bioclectrode to boost the signal obtained from the subject. The amplifier is not activated until the bioelectrode is in use. This activation may be started by removing a protective tape from the battery and circuitry.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings.